Standard sandbox portals currently have a 200,000 contact record limit as opposed to the full limit in the production portal. Custom objects on the other hand are permitted their full limit in sandbox. This poses a challenge when validating custom object records associated with contacts. Without the ability to also import all the assoicated contacts, it is not possible to test those associations and the related list and workflows in sandbox before going to production.
I think the sandbox portal limit for contact records whould be increased to accomodate the complexity of portal build-outs with custom objects.